Ophthalmic lenses, such as for example, soft contact lenses, can be fabricated by a variety of techniques. The more industrially popular methods involve the use of suitable liquid monomers, such as hydroxyethyl methacrylate (HEMA), that are deposited into especially configured mold halves. The monomer can be cured by any number of techniques, e.g. ultraviolet radiation, to form the lens. The liquid monomer, however, invariably contains dissolved gases such as oxygen and nitrogen, which can detrimentally affect curing either by forming unwanted bubbles, which can manifest as voids or other flaws in the final lens, or by otherwise interfering with the curing mechanism itself. It has thus become a common practice to degas the liquid monomer prior to use.
Accordingly, different techniques have been explored for degassing the monomer. One practice is to employ a rotary evaporator unit that removes excess gas from the monomer by rotating same under subatmospheric pressure. The container holding the mixture is then flushed with and held under nitrogen atmosphere. Another technique is described in U.S. Pat. No. 5,435,943 wherein the monomer is pumped through a gas permeable tube surrounded by a subatmospheric chamber. Gases in the monomer permeate the tube in favor of the lower pressure on the outside of same, the degassed monomer then being deposited into the lens molds and cured.
While these methods have proven commercially useful, efforts to advance the degas operation are nonetheless of interest. For example, the rotary evaporator method provides an opportunity for nitrogen gas to re-dissolve into the monomer during back fill flushing. Degas using permeable tubing has its own drawbacks: it typically demands batch operation due to the particulars involved in pumping viscous liquid monomer through tubing; thus, monomer is stored in vessels until required, whereupon it is sent to a tubular degas station which is off line. In a production environment where automation and advances in automation are critical, the use of a batch operation causes numerous inefficiencies that can adversely affect yield and logistics. Related to this is the fact that the permeable tubing is at some point subject to breakage, due for example to the pressure difference on either side of the tube wall and wear of the material of construction, typically silicone tubing. Breakage usually results in a shut down and often requires the wholesale replacement of tubing bundles, even those that are still intact, the arduousness of which can further disrupt operations. Moreover, the permeable tubing typically can not be properly cleaned when the need arises given, for example, the difficulty of cleaning the lumen. A batch degas operation also creates disposal problems inasmuch as liquid monomers for ophthalmic lenses commonly have shelf lives, and if not used within same they must be discarded. Moreover, in a batch operation where liquid monomer is pumped from a storage vessel, there is always some residual monomer remaining in the vessel after use. The aggregate amount of this residue, in the context of an industrial production facility, must be disposed of once past its shelf life.
There is thus a continuing need in the art for a degas technology that has increased effectiveness in both operation and maintenance, which technology can be employed in-line, and with reduced disposal issues.